PVA is short for polyvinyl alcohol. It is a water soluble material that can be used either as a release agent when applying gelcoat as a first layer into a mold, or to act as an air-inhibiting agent.
When used as a mold release agent, it can be sprayed or wiped on the mold, then left to dry until it looks clear and has a glossy appearance, before applying gelcoat to the mold. When it is used as an air-inhibiting surfacing agent, the PVA can be sprayed over tacky, unwaxed gelcoat (the gelcoat must be previously catalyzed and cross-linked and the styrene must have already evaporated).